Overlay (Not clickable and selectable window on top of fullscreen window) in WPF

So I basically want to make a transparent window that has a custom usercontrol (for example a dot inside of it) and display it TopMost. The only issue is that I can click it and then the fullscreen app loses focus, is there any way to prevent this? I already tried things like Focusable="False" and even DLL imports like SetWindowLong(hwnd, GWL_EXSTYLE, extendedStyle | WS_EX_TRANSPARENT);, but nothing seems to work.
can you try WS_EX_TRANSPARENT again but make sure its also WS_EX_LAYERED and you call SetLayeredWindowAttribute(hWnd, 0, 255, LWA_ALPHA);?